Take hold of a suitable weight barbell and lay face down on a bench inclined at around 30 - 45 degrees. Your core is tight and your toes are planted firmly on the ground. Your arms hang down outstretched on the sides of your body, taking the weight of the barbell in a shoulder-width, palms up grip. This is the starting position.
Commence the movement by curling the barbell up from the elbow joint and squeezing your biceps. The elbows should work like a hinge with the forearm being the only moving part. Exhale through the movement. Do not use momentum to throw the dumbbells up.
When your biceps are at maximum contraction, squeeze for a second and then reverse the movement by extending the arms to the starting position, inhaling as you do.